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RESEARCH PRODUCT

GW190521 as a Merger of Proca Stars: A Potential New Vector Boson of 8.7×10−13  eV

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Advanced LIGO-Virgo have reported a short gravitational-wave signal (GW190521) interpreted as a quasicircular merger of black holes, one at least populating the pair-instability supernova gap, that formed a remnant black hole of ${M}_{f}\ensuremath{\sim}142\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ at a luminosity distance of ${d}_{L}\ensuremath{\sim}5.3\text{ }\text{ }\mathrm{Gpc}$. With barely visible pre-merger emission, however, GW190521 merits further investigation of the pre-merger dynamics and even of the very nature of the colliding objects. We show that GW190521 is consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars), forming a final black hole with ${M}_{f}=23{1}_{\ensuremath{-}17}^{+13}\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, located at a distance of ${d}_{L}=57{1}_{\ensuremath{-}181}^{+348}\text{ }\text{ }\mathrm{Mpc}$. This provides the first demonstration of close degeneracy between these two theoretical models, for a real gravitational-wave event. The favored mass for the ultralight vector boson constituent of the Proca stars is ${\ensuremath{\mu}}_{\mathrm{V}}=8.7{2}_{\ensuremath{-}0.82}^{+0.73}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\text{ }\text{ }\mathrm{eV}$. Confirmation of the Proca star interpretation, which we find statistically slightly preferred, would provide the first evidence for a long sought dark matter particle.